How can I see my nvidia quadro 6000 in 2.80, 2.81 CUDA?
The requirements for GPU rendering are higher than the minimum requirements for running Blender.


  Supported Graphics Cards
  
  These requirements are for basic Blender operation, rendering using the GPU requires slightly different requirements.


The manual states that CUDA compute capability 3.0 or higher is required for GPU rendering. Graphics cards that do not meet that requirement will not be listed in the preferences.


  NVIDIA CUDA
  
  NVIDIA CUDA is supported for GPU rendering with NVIDIA graphics cards. Blender supports graphics cards with compute capability 3.0 and higher. To make sure your GPU is supported, see the list of NVIDIA graphics cards with the compute capabilities and supported graphics cards.
  
  NVIDIA CUDA GPU rendering is supported on Windows, macOS, and Linux.


Since the Quadro 6000 has compute capability 2.0 it's not supported for GPU rendering.